20 THE VOYAGE OF H.M.S. CHALLENGER. 



never take a vertical course leading from the depths of the coral to the surface. A free 

 vertical communication is, however, established by the smaller vessels (PL XIII. fig. 5). 

 In the thin films of Milkpora alcicornis the trabecule of hard tissue run with remarkable 

 uniformity in straight lines parallel to one another, whilst the main canals cross them 

 with a serpentine course. 



Histology of the Ccenosteum. — In histological structure the hard tissue composing the 

 ccenosteum of Millepora seems to resemble closely that of the coralla of Heliopora and 

 most Anthozoa. It is composed of lamellae of fibro-crystalline calcareous matter (PL XIII. 

 fio-. 8), the fibres of the superposed lamellae crossing one another at all angles in the mass. 

 In some places, in thin sections of the ccenosteum, the appearance shown at a (PL XIII. 

 fig. 8) is clearly to be seen. The calcareous fibres of the hard tissue terminate towards a 

 cavity in the ccenosteum as a series of short points, seeming to show a composition of the 

 hard tissue out of definite rod-like elements. Such an appearance is only to be met with 

 sparingly, and possibly occurs at spots where the ccenosteum was in active growth. The 

 hard tissue is bored in all directions by parasitic vegetable organisms (PL XIII. figs. 

 6 and 8). 



Chemical Composition of the Ccenosteum. — Although the animals forming the ccenos- 

 teum of Millepora differ so widely from those by which all other corals except the 

 Stylasteridse are secreted, their ccenostea appear to agree in chemical composition with 

 those of other corals as closely as they do in histological structure. Analyses of the 

 ccenostea of two species of Millepora are given by Professor Dana. One is an analysis 

 of Millepora tortuosa from the Fijis, by Mr Silliman, Jim. 1 The composition was found 

 to be as follows : — 



Carbonate of lhne . . . .94-226 



Phosphates and fluorides . . . T200 



Organic matter . . . . . 4 - 574 



Mr S. P. Sharpies 2 found the ccenosteum of Millepora alcicornis to consist of — ■ 



Carbonate of lime . . . . 97 '4 G 



Phosphate of lime . . . . . 0'27 



Water and organic matter . . . 2'4 



There is no marked difference between these results and those obtained from 

 Anthozoan corals. 



1 Structure and Classification of Zoophytes, by J. D. Dana, A.M., Philadelphia, Lea and Blanchard, 1846, 

 Appendix, p. J 30. 



2 Corals and Coral Islands, by J. D. Dana, London, 1872, p. 105. 



